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Ovulation, Fertilisation & Implantation — Comprehensive Reproductive Health Notes

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PART 1: OVULATION

1.1 Hormonal Control of the Ovarian Cycle

• Follicular phase (Days 1–13): FSH stimulates cohort of primordial follicles; one becomes the dominant (Graafian) follicle, grows to ~25 mm. Rising estrogen from granulosa cells initially exerts negative feedback on FSH/LH.
• Pre-ovulatory estrogen peak: When estrogen exceeds a threshold (~200 pg/mL for ≥50 hours), it switches to positive feedback, triggering the LH surge.
• LH surge: LH rises 6–10 fold; peaks ~16 hours before ovulation. FSH also rises 2–3 fold synergistically.
• Ovulation timing: Occurs 24–36 hours after the LH peak, 10–12 hours after the LH peak in some sources.

1.2 Follicular Maturation Leading to Ovulation

1. Completion of meiosis I → secondary oocyte + first polar body
2. Arrest at metaphase II (meiosis II only completed at fertilization)
3. Increased collagenase activity → digestion of collagen in follicular wall
4. Rise in prostaglandins → smooth muscle contraction of the ovarian wall

1.3 Mechanism of Follicle Rupture

1. A small avascular spot — the stigma — appears at the apex of the bulging follicle
2. Lysosomal proteolytic enzymes (collagenase, plasmin, matrix metalloproteinases) digest the follicular capsule wall
3. Prostaglandins cause vasodilation and hyperemia → plasma transudation into the follicle
4. Follicle swells; the stigma balloons outward, then ruptures (~2 minutes after ooze begins)
5. The viscous antral fluid expels the secondary oocyte surrounded by the corona radiata (radially arranged cumulus oophorus cells) and zona pellucida — together forming the oocyte–cumulus complex
6. MAPK3/1 (ERK1/2) signaling pathways in follicular cells also help regulate this process

1.4 The Oocyte Post-Ovulation

• The released cell is a secondary oocyte (not a mature ovum) — still arrested in metaphase II
• Surrounded by: zona pellucida (glycoprotein coat: ZPA, ZPB, ZPC) → corona radiata → cumulus oophorus remnant
• Swept into the fallopian tube (uterine tube) by ciliary action; must be fertilized within 12–24 hours
• If not fertilized, degenerates

1.5 Corpus Luteum Formation

• Remaining granulosa and theca interna cells luteinize → enlarge 2× diameter, fill with lipid → yellow appearance
• Form the corpus luteum, which secretes progesterone (primarily) and estrogen
• Progesterone causes the secretory transformation of the endometrium, preparing it for blastocyst implantation

1.6 Clinical Correlates of Ovulation

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PART 2: FERTILISATION

2.1 Site and Timing

• Fertilisation normally occurs in the ampulla of the uterine tube (widest part, closest to the ovary)
• Window: oocyte viable 12–24 hours post-ovulation; sperm viable in female tract up to 5–7 days
• Of ~200–300 million spermatozoa deposited in the vagina, only 300–500 reach the site of fertilisation; only one fertilises the oocyte

2.2 Sperm Transport

1. Only ~1% of deposited sperm enter the cervix
2. Transport from cervix to uterine tube via uterine smooth muscle contractions (not primarily sperm motility); can take 30 minutes to 6 days
3. Sperm reach the isthmus → become less motile and pause
4. At ovulation, cumulus cell chemoattractants stimulate sperm to resume active motility toward the ampulla

2.3 Capacitation

• Removal of a glycoprotein coat and seminal plasma proteins from the membrane overlying the acrosomal region
• Membrane becomes destabilized → primes sperm for acrosome reaction
• Only capacitated sperm can penetrate the corona radiata and undergo the acrosome reaction

2.4 Acrosome Reaction

• Outer acrosomal membrane fuses with the overlying plasma membrane → releases acrosomal enzymes
• Key enzymes released: hyaluronidase, acrosin (proteolytic), esterase, neuraminidase
• These digest a path through the zona pellucida

2.5 Three Phases of Fertilisation

Phase 1 — Penetration of the Corona Radiata

• Capacitated sperm pass freely through the corona cells
• Hyaluronidase (from acrosome) + tubal enzymes disperse follicular cells
• Sperm tail movements aid mechanical penetration

Phase 2 — Penetration of the Zona Pellucida

• Acrosin (serine protease) and other enzymes lyse the zona, forming a pathway
• Once one sperm penetrates, the zona reaction occurs immediately:
  • Cortical granules beneath the oocyte membrane release lysosomal enzymes into the perivitelline space
  • These enzymes modify the zona pellucida, making it impermeable to further sperm
  • This is the primary block to polyspermy

Phase 3 — Fusion of Plasma Membranes

• Sperm plasma membrane (posterior head region — acrosomal head cap membrane is shed) fuses with oocyte membrane
• The sperm head and tail enter oocyte cytoplasm; sperm plasma membrane remains on oocyte surface
• Oocyte responds with:
  1. Cortical/zona reaction (as above)
  2. Resumption of meiosis II → second polar body extruded → female pronucleus formed (22+X)
  3. Metabolic activation of the egg → initiates early embryogenesis

2.6 Pronuclei and Syngamy

• Sperm nucleus decondenses → swells → male pronucleus (22+X or 22+Y); tail detaches and degenerates
• Male and female pronuclei migrate toward each other; both haploid, each must replicate their DNA
• Nuclear envelopes break down; chromosomes align on the first mitotic spindle
• Chromosomes split at centromere; sister chromatids move to opposite poles
• Deep cleavage furrow divides cytoplasm → 2-cell zygote (diploid, 46 chromosomes)
• The entire fertilisation process takes approximately 24 hours

2.7 Results of Fertilisation (Summary)

2.8 Sex Preselection

• X-bearing sperm carry 2.8% more DNA than Y-bearing sperm
• Techniques to separate X/Y sperm exploit: differential swimming speed, electrophoretic migration, DNA content differences (flow cytometry)
• Selected sperm can be used in IVF to achieve sex selection

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PART 3: IMPLANTATION

3.1 Pre-implantation Development (Zygote to Blastocyst)

• Trophoblast (TE) — outer single-cell layer; will form placenta and membranes
  • Polar trophectoderm — overlies inner cell mass; initiates implantation
  • Mural trophectoderm — remainder of outer shell
• Inner cell mass (ICM/embryoblast) — gives rise to the embryo proper
• Blastocoel — fluid-filled cavity

3.2 Endometrial Receptivity — The "Window of Implantation"

3.3 Stages of Implantation

Stage 1 — Apposition

• Loose, initial contact between polar trophectoderm and luminal endometrial epithelium (typically posterior uterine wall)
• Reversible at this stage

Stage 2 — Adhesion

• Firm adhesion mediated by:
  • HB-EGF (endometrium) binding ErbB1/ErbB4 (on polar TE microvilli/podosomes)
  • Integrins on TE surface interacting with endometrial extracellular matrix (fibronectin, laminin)
  • LIF, blastocyst-to-endometrium signals
• Trophoblast microvilli interdigitate with endometrial surface

Stage 3 — Invasion (Penetration)

• Polar TE cells break through the endometrial epithelial barrier
• Form a cytotrophoblast shell around the embryo
• Cytotrophoblast cells fuse → primitive syncytiotrophoblast (invasive + secretory)
• By Day 10–11: embryo is completely embedded in the endometrial stroma

3.4 Trophoblast Differentiation

CTB progenitor cell
      │
      ├──→ SYNCYTIOTROPHOBLAST (STB)
      │    • Multi-nucleated
      │    • Covers placenta
      │    • Transport + endocrine functions
      │    • Produces hCG, hPL, estrogens, progesterone
      │
      └──→ EXTRAVILLOUS CYTOTROPHOBLAST (EVT)
           │
           ├──→ Interstitial EVT (iEVT)
           │    • Invades endometrial stroma
           │    • Penetrates superficial 1/3 myometrium
           │
           └──→ Endovascular EVT (eEVT)
                • Invades maternal arterioles
                • Replaces arterial endothelium
                • Vascular remodeling → low-resistance, high-capacitance flow

• Physiologic hypoxia (<30 mmHg) → HIF stabilization
• ASCL2 (helix-loop-helix factor) → promotes EVT, inhibits STB
• Canonical WNT/β-catenin (TCF4) signaling
• Integrin switch: loss of α6/β4 → gain of α5/α1 (fibronectin receptors)
• MMPs (MMP-9) and plasminogen activators enable stromal invasion

3.5 Decidualization

• Stromal cells enlarge, become glycogen-rich, rounded
• This transformation is regulated by HOXA10, HOXA11, HAND2, BMP2
• The decidua provides nutritional support for the early embryo and modulates immune tolerance

3.6 hCG and Rescue of the Corpus Luteum

• The syncytiotrophoblast begins secreting human chorionic gonadotropin (hCG) at implantation (~Day 6–8)
• hCG peaks at ~10 weeks gestation, then falls
• hCG acts like LH → maintains the corpus luteum of pregnancy → continuous progesterone/estrogen production prevents menstruation
• The corpus luteum is essential for the first ~20 weeks until the placenta assumes steroidogenesis
• hCG is the basis of all pregnancy tests

3.7 Normal Implantation Site

• Normally occurs on the posterior wall of the uterine body (endometrium)
• The most common site is the upper posterior uterine wall

3.8 Clinical Correlates of Implantation

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INTEGRATED TIMELINE: From Ovulation to Implantation

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KEY HORMONES SUMMARY TABLE

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